Video signals are commonly defined in terms of a fixed set of primaries, such as those defined by the European Broadcast Union (EBU). In practice, however, the reproduction gamut may differ from that of the input signal, such that gamut-mapping algorithms are required to establish the optimal drive values for display of the video signals. This problem occurs frequently for mobile displays, which typically have a gamut that is substantially narrower than that of the EBU-standard.
The most straightforward approach to gamut mapping uses the RGB-drive values of the input directly as drive values for the primaries of the reproduction gamut without any color processing. However, in practice this may result in highly de-saturated images.
To better maintain the image colors, true-color mapping is applied to the input signal followed by a clipping operation to map the out-of-gamut colors to the required range (of [0, 255] for a non-normalized eight bit representation, or [0, 1] for a normalized representation). Because image colors positioned within the reproduction gamut are preserved, the overall colorfulness of the image is higher than that obtained by ignoring the color processing altogether. True-color mapping is for example disclosed in Wyszecki, G. and Stiles, W. S., “color science: concepts and methods, quantitative data and formulae (2nd edition), John Wiley & Sons, Inc., New York 1982.
Unfortunately, clipping results in highly annoying loss of detail. Moreover, when applied to individual RGB-channels, clipping may introduce hue errors, for which the human eye is very sensitive. The occurrence of hue errors can be partly prevented by dividing out-of-gamut colors by the maximum drive value of the RGB sub-pixels (which is also referred to as clipping-to-black or mapping to black). Although this approach works satisfactorily for images with only a few out-of-gamut pixels, it still causes a loss of detail when large areas of the image are located outside of the reproduction gamut. To prevent the occurrence of clipping artifacts, the gamut-mapping algorithm can be combined with an overall brightness reduction (soft clipping) as well as a saturation-dependent brightness correction.